Conventionally, an amplifier is used in a wireless communication device. It is generally known that the efficiency of an amplifier is highest in an output saturation state (that is, in a non-linear state). In a non-linear state, a distortion tends to occur in an output signal of an amplifier, and the distortion might cause deterioration of a transmission performance. Thus, there are cases where, in order to increase both of the transmission performance and efficiency of the amplifier, a distortion compensation unit that compensates for a “distortion” originated from a power amplifier is provided in a wireless communication device. Note that, when amplifiers having about the same saturation power are compared, a “distortion” tends to occur when the power of an input signal to an amplifier, that is, the amplitude of a transmission signal, is large.
The distortion compensation unit compares, for example, a transmission signal that has been converted into two series, that is, an I signal and a Q signal by an S/P converter and a signal obtained by feeding back a part of a signal that has been converted into an I signal and a Q signal and then has been subjected to amplification processing to each other, and computes a distortion compensation coefficient such that a difference between the signals is zero. Then, the distortion compensation unit performs distortion compensation by multiplying the transmission signal that has been converted into an I signal and a Q signal by the distortion compensation coefficient. The distortion compensation unit compares the transmission signal before distortion compensation and a signal obtained by feeding back a part of a signal that has been amplified after the distortion compensation to each other, and updates the distortion compensation coefficient as desired. That is, in a predistortion method, which is a distortion compensation method, a characteristic that is inverse to a distortion characteristic of an amplifier is added to an input signal to the amplifier in advance. Thus, in an output of the amplifier, a signal with less distortion is obtained.
In recent years, in a wireless communication device, a gallium nitride (GaN) device, which is an amplifier with high output power and high power efficiency, may be used.
Japanese Laid-open Patent Publication No. 2012-249022 discusses the related art.